From Wikipedia, the free encyclopedia

In physics,
absorption of electromagnetic radiation is the way
by which the energy of a photon is taken up by matter,
typically the electrons of an atom. Thus, the electromagnetic
energy is transformed to other forms of energy for example, to
heat. The absorption of light during wave propagation is often called attenuation. Usually, the absorption of
waves does not depend on their intensity (linear absorption),
although in certain conditions (usually, in optics), the medium changes its transparency
dependently on the intensity of waves going through, and the Saturable
absorption (or nonlinear absorption) occurs.

Absorbance (also
called "optical density") and optical thickness
(also called "optical depth") are two related measures of the total
light-blocking power of a certain medium with a certain
thickness.

Percentage of the incoming light which gets absorbed

All these quantities measure, at least to some extent, the same
thing: How well a medium absorbs radiation. However, practitioners
of different fields and techniques tend to conventionally use
different quantities drawn from the list above. Fortunately, it is
easy to convert from one measure to another, see Mathematical
descriptions of opacity.

Measuring
absorption

The absorbance of
an object quantifies how much of the incident light is absorbed by
it (not all photons get absorbed, some are reflected or refracted instead). This may be related to
other properties of the object through the Beer-Lambert law.

In medicine, X-rays
are absorbed to different extents by different tissues (bone in particular), which is the
basis for X-ray imaging.

In chemistry and materials
science, different materials and molecules will absorb
radiation to different extents at different frequencies, which
allows for material identification.

In optics, sunglasses,
colored filters, dyes, and other such materials are designed
specifically with respect to which visible wavelengths they absorb
and how much.

In biology,
photosynthetic organisms require that light of the appropriate
wavelengths be absorbed within the active area of chloroplasts, so that
the light energy can be converted into chemical energy within
sugars and other molecules.